A switching power supply apparatus using nonlinear control (for example, hysteresis window control, fixed on-time with bottom detection control, fixed off-time with upper detection control or the like) can obtain a high load response characteristic with a simple circuit configuration as compared to a switching power supply apparatus using linear control (for example, voltage mode control, current mode control or the like).
Such a switching power supply apparatus using nonlinear control may be configured to use an output ripple voltage (a ripple component of an output voltage) to drive a comparator and perform a switching control of an output transistor. The output ripple voltage is required to have a relatively large amplitude (wave height) in order to detect the output ripple voltage accurately. Therefore, an output capacitor having a relatively large equivalent series resistance (ESR) (for example, a conductive polymer type capacitor) needs to be used in the switching power supply apparatus, which may lead to limited part selection and increased cost.
In the related art, there is known a technique (a ripple injection technique), in which a ripple component is injected into (superimposed on) a reference voltage REF or a feedback voltage FB which are input to a comparator in a forced manner to stably drive the comparator.
FIG. 10 is a circuit diagram showing an example of a conventional switching power supply apparatus employing such ripple injection technique. The conventional switching power supply apparatus employs a method of injecting a ripple component into a feedback voltage FB via a resistor-capacitor (RC) circuit using a square wave-shaped switch voltage SW (hereinafter referred to as a RC injection method). If this ripple injection technique is employed, stable switching control may be implemented even if the amplitude of an output ripple voltage is not very large, which makes it possible to use a stacked ceramic capacitor of a small ESR as an output capacitor.
However, in a switching power supply apparatus employing the RC injection method, since the ripple component superimposed on the feedback voltage FB has a waveform obtained by smoothing a square wave-shaped switch voltage SW, there are problems in that (1) an amplitude (wave height) of the ripple component is varied depending on an ON time of the switch voltage SW (see FIG. 11), and (2) linearity of the ripple component is damaged depending on the ON time of the switch voltage SW (see the dashed line in FIG. 11).
In addition, in the switching power supply apparatus employing the RC injection method, since a ripple current component of an inductor is not included in the ripple component superimposed on the feedback voltage FB, there is another problem in that (3) ringing of an output voltage OUT is likely to appear due to insufficient stability in the event of a sudden change in a load (see FIG. 12).